In general, in image-forming apparatuses adopting electrophotographic-method, such as a printer, a copy machine, and a facsimile machine, by forming a developing-material image (a toner image) on a printing material, and by melting and fixing the toner image, through heating and pressure processing, on the printing material, an image is formed.
Meanwhile, the types of printing materials utilized for these image-forming apparatuses include a wide variety of materials such as normal paper, high-quality paper onto which special surface processing is applied, resin-made sheets for OHPs. Furthermore, because the image-forming apparatuses have spread all over the world, the types of printing materials utilized for image-forming have rapidly been increasing in number. Therefore, the image-forming apparatuses are expected to be able to form good images with various types of printing materials being utilized in each region.
Thermal-resistance difference due to difference in surface property exists between a printing material, to be used, having a smooth surface (referred to as smooth paper, hereinafter) and a printing material having a rough surface (referred to as rough paper, hereinafter). Heating efficiency from a heating source in a heat-fixing unit to the surface of a sheet of paper differs depending on the thermal-resistance difference. For example, even though fixing is applied to rough paper at a temperature appropriate to smooth paper, insufficient fixing is caused. This is because fixing to rough paper requires a higher temperature than that required by fixing to smooth paper. Therefore, in current apparatuses, a temperature at which a toner image can sufficiently be fixed even on rough paper is utilized as a standard fixing temperature.
However, with these apparatuses, fixing to smooth paper is always implemented at excess temperature; therefore, a hot-offset problem occurs. Furthermore, the fixing temperature is too low for paper that is rougher than rough paper, whereby a problem of defective fixing also occurs. A further higher temperature is required for such paper. Conventionally, utilizing such paper has inconvenienced the user, because the user has to manually change the setting for fixing temperature.
In addition, as a fixing apparatus that is provided in an image-forming apparatus adopting the electrophotographic-method, so-called heat-roller-system heat-fixing units have widely been utilized. In the heat-roller system, by making a printing material carrying a non-fixed toner image pass through a nip portion, the toner image is fixed as a permanent image on the printing material. The nip portion is formed with a fixing roller and a pressure roller that rotate being pressed by each other.
Meanwhile, from the recent viewpoint of energy-saving promotion, a fixing method has been proposed, in which, without supplying a fixing unit in a standby mode with electric power, power consumption is suppressed as much as possible. In this method, a system in which a toner image on a printing material is fixed through a small thin film, having small heat capacity, interposed between a heater portion and a pressure roller, i.e., a so-called film-heating system, has been employed (Japanese Patent Laid-Open No. 63-313182, No. 2-157878, and No. 4-44074).
A fixing unit employing the film-heating system has been drawing attention, because of its higher heat-transfer efficiency and shorter start-up time than those of units employing the heat-roller-system. In addition, the film-heating system has been applied also to high-speed models.
However, in this system, heat-up speed is emphasized; thus, it is necessary to diminish the heat capacity of the heating surface of a fixing portion. Making the heat capacity of the heating surface small hinders the formation of an elastic layer on the heating surface. Therefore, in effect, a hard heating surface has been utilized. If the heating surface is hard, difference in heating efficiency is liable to occur, due to unevenness of the surface of a printing material.
Therefore, a method has been proposed, in which the fixing temperature is automatically switched to an optimal temperature, by detecting the heat capacity and the surface roughness, of a printing material (Japanese Patent Laid-Open No. 7-230231). Specifically, by measuring through a non-contact temperature-detecting sensor the temperature of a printing material, the fixing temperature is set to an optimal value, based on the measured temperature. Accordingly, for thin paper, which is readily heated, by reducing the fixing temperature, a curl and a hot-offset can be prevented. In addition, in the case of a printing material having a rough surface, or thick paper, by raising the fixing temperature, sufficient fixing ability can be obtained.
However, in the foregoing related arts, because a non-contact temperature sensor is utilized, the temperature of a printing material can not accurately be detected. This is because the surface of the non-contact temperature sensor is fogged with steam. The steam is produced because, when the printing material is heated and fixed, moisture included in the printing material is concurrently heated.
It is assumed that, by forming an air path and the like, by means of a fan, steam does not fog the surface of the non-contact temperature sensor. In this case, a new defect may be caused, in which the air path also affects the surface temperature of the printing material. For that reason, the method, of determining types of printing materials, that utilizes a non-contact temperature sensor such as an infrared-ray sensor has not been practiced in effect.
Therefore, it is an object of the present invention to solve such and other issues. In addition, other issues may be understood by reading through the entire specification.